Diagnostic systems analyzing body fluids such as whole blood, plasma, serum, urine, etc., use test carriers (such as test strips, cassettes etc.) to take up a fluid sample to be examined. Such test carriers usually have a sample application site and a measuring chamber that is spatially separated therefrom. The structure of the test carrier must ensure that the sample is transported from the sample application site to the measuring chamber and that the latter is adequately filled with sample liquid. Capillaries can for example be used for this liquid transport as they are described for example in WO 03/095092 or WO 2004/113917. The measuring chamber usually comprises a detection zone which has a defined geometry and a defined surface. Deviations in the dimensions lead to deviations in the measuring result.
Plastic materials such as foils or injection molded parts which enable a cost-effective manufacture are usually used to manufacture the test carriers. These plastic materials must be joined together during production by adhesively bonding, welding or injection molding processes. In doing so there is a risk that the geometry of the detection zone in the measuring chamber deviates from the specifications for individual test carriers. These deviations can occur during manufacture for example as a result of adhesive exuding into the measuring chamber during manufacture or they may be due to welding or bonding conditions that deviate from the specifications. It is, however, also possible that test carriers become deformed due to mechanical or thermal stress after manufacture resulting in a change in the detection surface in the measuring chamber. Furthermore, it is conceivable that the electrode surface itself was not manufactured in the intended geometric dimensions or was changed by subsequent damage.
Finally, the detection surface can be altered by incomplete filling with sample liquid, by the inclusion of air bubbles in the sample liquid or by a leaky measuring chamber.
In order to check for manufacturing defects in electrochemical blood glucose sensors that run on direct voltage or to check for inadequate covering of the electrodes with sample, U.S. Pat. No. 6,733,655 (Davies et al.) proposes that two independent working electrodes are provided on a test strip which together with a reference electrode that is used in common, result in two sensors. The two sensor parts are measured with respect to the concentration of the substance in the sample (thus in the case of glucose sensors the glucose content of the sample is measured twice in parallel) and the two measured values are compared with one another. If both measured values are the same, it is assumed that the sensor is basically all right. If the measured values differ considerably than an error is assumed.
A disadvantage of this method is that the actual measurement process is used as a control. Thus especially in the case of lengthy measurements (e.g., in the field of coagulation diagnostics) it may take a relatively long time until it is known whether a sensor is OK or not. Defects in parts of the measurement set-up that are identical for both sensor channels (e.g., scratches in the reference electrode etc.) are not detected since they have equal effects on both channels.
U.S. Pat. No. 5,352,351 (White et al.) describes methods for determining the coverage of a measuring surface by sample liquid in electrochemical blood glucose sensors and for monitoring the measuring process. For this purpose discrete direct voltages which differ with respect to time are applied to the electrodes of corresponding sensors and conclusions are drawn from the measured currents.
The method according to U.S. Pat. No. 5,352,351 has basically the same disadvantages as U.S. Pat. No. 6,733,655.
In the prior art there is a lack of methods which would enable the above-mentioned problems to be reliably detected and thus avoid erroneous measurements or to flag such measurements as erroneous.